Among all the fields of international research on giant pandas, breeding is the most prominent, experts said.

Since cooperative research started in 1994, the Chengdu Research Base of Giant Panda Breeding has helped produce 13 cubs in Japan (11 survived), three in the United States and twins in Spain. The China Conservation and Research Center for the Giant Panda in Sichuan province contributed to the breeding of six cubs in the US, two in Austria and one in Thailand.

Pandas don't live on bamboo alone. Zhang Le feeds carrots to 1-year-olds at Bifengxia panda base in Sichuan. [China Daily]?

Any baby born to pandas on loan from China belongs to China, and the offspring are sent back at about age 4 to become part of the breeding population. So far, 11 of the pandas born overseas have returned.

In the process, the cooperative ventures have helped China raise its own level of animal breeding, feeding and research.

Different techniques

In 2006, Zoo Atlanta invited Hou Rong, director of a research center at Chengdu's research base, to the US in spring to help a pair of pandas breed via artificial insemination. The zoo had tried three times, unsuccessfully, to collect semen from Yang Yang.

A panda keeper later found that Yang Yang had a retrograde ejaculation problem. His semen was redirected to the urinary bladder rather than being ejaculated via the urethra. Hou suggested that semen be collected by electro-ejaculation combined with rectal massage.

A different procedure was suggested by another expert the zoo had invited - Richard Fayrer-Hosken, an animal reproductive medicine specialist with the University of Georgia's College of Veterinary Medicine. He insisted on first using a pipe to extract semen from the panda's urinary bladder and then washing urine off the sample.

After rounds of discussion, both experts succeeded in collecting sperm from Yang Yang in their own ways. But the quality of Hou's sample was considered better because the sperm had not been stained by urine.

Two 4-month-old panda cubs have fun in their nursery in Bifengxia center in Ya'an, Sichuan province. [China Daily]?

The zoo used the fresh sperm sample collected by Hou to inseminate Lun Lun, who gave birth to Mei Lan on Sept 6, 2006. Mei Lan, whose name means "Atlanta's Beauty", returned to China in February 2010 and was appointed the global ambassador for Earth Hour, an event led by the WWF to fight climate change.

Hou used the same method in 2007 to help the Memphis Zoo in the US. The zoo had tried to collect sperm from a panda named Le Le five times but to no avail. When Hou succeeded, all staff workers on the scene burst into cheers and applause.

"After I was done," she said, "an American panda keeper followed me back to the laboratory. At that time, he didn't know I speak English and bowed to me deeply. I was so impressed."

Mothering lessons

Collecting sperm is not the only critical process. So is timing.

The female's estrus period comes just once a year, and she is receptive for only a few days. Therefore, it is crucial to seize the right moment and frequency of pairing, said Huang Zhi, animal management director of China Conservation and Research.

When mating succeeds, the job still may not be done. The center also has sent experienced staff members abroad to help female pandas raise their cubs. Some of the mothers do not do well because they have health problems, are inexperienced or can handle only one cub at a time (twins are not rare).

Chinese experts have taught their foreign colleagues how to bottle-feed and monitor cubs and how to feed the mother panda after delivery.

Getting to know you

Zoo Atlanta had received Yang Yang and his mate, Lun Lun, from the Chengdu base in 1999. That is also when the facilities began cooperating to study animal behavior, something no employee of any Chinese zoo had heard of before.

Many scientists and technicians at the Chengdu Zoo even questioned whether such research deserved their time and effort. They later found its value.

Statistics in the 1990s showed that among more than 100 pandas raised over the years in a simulated wild environment, only seven males had been able to mate naturally.

If a female panda at Chengdu Zoo entered estrus, the zoo would borrow a male from Beijing Zoo immediately and put them together. Contrary to their keepers' expectations of a happy couple, the two would fight ferociously. Nobody could explain why.

Through behavior studies, researchers learned that pandas need sufficient information about each other before mating. In the wild, they leave scent marks with urine and glands on their buttocks. With these marks, a male can tell a female his personal information and find the right partner.

Based on this research, panda keepers now put the male and female in adjacent enclosures before her estrus cycle to let them get to know each other. Staff members stop washing out the enclosures, preserving the scent marks left by the pandas.

Now, Hou said, 60 percent of adult male pandas worldwide have natural mating experiences. (The remaining 40 percent are used as sperm donors.) About half of pandas mate naturally every year, up from 10 percent in the 1990s. The annual breeding success rate also has doubled, reaching nearly 60 percent on average.

'We have learned a lot'

"China's ability to research giant pandas has substantially improved compared to 20 years ago," Hou said. "Like other Third World countries, our level of medical care, animal protection and scientific research used to remain low, but we have learned a lot through international cooperation all these years.

"Today, our country has reached the international level in terms of animal exhibition, research, feeding and management. We absorbed the advanced technologies of other countries, applied them into our animal breeding practice and made our own renovations."

Based on her cooperation with the University of Liverpool and the National Cancer Institute in the US, Hou and her colleagues built their own research team, developed new technology to standardize paternity testing for giant pandas, and applied for a national patent in 2010.

Tracking family lines is critical to maintaining genetic diversity in the endangered panda species.